Char forming or intumescent flame retardant compositions are well known in the painting, coating and plastics fields. These systems generally consist of three components: an acid generating component, a char forming component and a blowing agent. The theory and background of such intumescent flame retardant systems are widely reported in the literature; see, for example, The Chemistry and Uses of Flame Retardants, J. Lyons, (Wylie-Interscience, New York, N.Y., 1970) pages 256-280; "Intumescent Coating Systems, Their Development and Chemistry", H. L. Vandersall, J. Fire & Flammability, Volume 2, pages 97-140, (April, 1971).
Vandersall describes the commercial use of ammonium polyphosphate (APP) as an acid generator, dipentaerythritol (DPE) as a char former, and melamine (ME) as a blowing agent in a system for painting and coating applications. U.S. Pat. No. 3,810,862 to Mathis et al. describes using an APP/DPE/ME system in plastics. U.S. Pat. No. 4,198,493 to Marciandi describes an intumescent flame retardant system for use in paints and plastics in which the polyhydroxy component DPE is replaced by the polyhydroxy compound tris(2-hydroxyethyl) isocyanurate (THEIC).
U.S. Pat. No. 4,727,102 to Scarso discloses a flame retardant polymer composition which appears to be similar to U.S. Pat. No. 4,198,493 and describes one embodiment comprising a mixture of APP, melamine cyanurate (MC) and a hydroxyalkyl derivative of isocyanuric acid wherein the derivative of isocyanuric acid is in the form of a homopolymer. Additionally, U.S. Pat. No. 4,727,102 discloses (see column 2, lines 48-56) a polypropylene polymer containing a two-component flame-retardant additive system consisting of 65% polypropylene, 15% APP and 20% THEIC (based on weight percentages), wherein the polypropylene polymer with the flame-retardant system has a VO, UL94 rating (see explanation of UL94 test and rating method below) with poor mechanical and thermal characteristics.
Flame retardant polymer compositions based upon halogenated organic compounds, phosphorous containing organic compounds, and halogenated organic compounds/antimony oxide mixtures are well known. These systems suffer from several disadvantages, however, that make their use undesirable. The addition of these types of flame-retardant additives to polymers reduces the physical properties of the polymers, such as tensile strength, impact strength, flexural strength and elongation. Additionally, because of environmental and safety factors, it is desirable to get away from systems employing halogenated components.
One of the factors complicating the search for suitable flame retardant additives is the different behavior and properties of the various materials to be rendered flame retardant. This includes the need for different loading levels of flame retardant additives depending on the type of polymer system being considered. For example, polyolefins may require loadings of 30-50 weight percent, polyamides may require loadings of 20-25 weight percent and thermoplastic polyesters may only require loadings of 12-20 weight percent. Depending on the polymer system used and the type and amount of flame retardant used, the physical and mechanical properties of the polymer system may be adversely impacted.
Another problem that has been investigated in this area is the problem of finding suitable drip retardants (also called drip suppressants). It is known that flaming polymer droplets increase the spread of fire and efforts have been made to produce flame retardant polymer compositions which do not exhibit this flame spreading tendency. Many of these attempts at finding drip suppressants have been based on halogenated or phosphorous-organic compound type flame retardant systems, perhaps because the intumescent type of flame retardant systems are generally more drip suppressing than those which are not intumescent.
H. L. Vandersall (at pages 129-130 of the reference discussed above) discloses that the quality of an intumescent foam generated by an intumescent flame retardant system can be improved by the incorporation of finely divided solids into the system. Such particles are believed to assist in the "nucleation" of the foam.
Other references describe improved flame retardant polymer compositions obtained by the use of additives such as fiber glass (see U.S. Pat. No. 4,490,504 to Mark), or polytetrafluoroethylene powder (see U.S. Pat. No. 4,344,878 to Dolce; U.S. Pat. No. 4,107,232 to Haaf, et al; and U.S. Pat. No. 3,671,487 to Abolins). These references describe polymer compositions containing halogen or phosphorous or similar types of materials.
Additionally, problems have been encountered in the processability of polymers to which flame retardants have been added. For example, in flame retardant polymers containing only APP and THEIC two particular processing problems are screw slippage and mold plate-out (also called mold deposit or bloom). THEIC melts at about 140 degrees C, but is incompatible with most polymer systems. Because of its low melt temperature it is believed that the THEIC acts as a plastizer in the polymer melt. The viscosity of THEIC in such a system is about 1 centipoise. The THEIC migrates to the outside of the melt surface while it is being processed in the extruder. Since the extruder relies on friction of the melt with the metal barrel, and the THEIC reduces the friction between the melt and the metal barrel to almost zero, screw slippage occurs. Also, the migration of THEIC to the surface of the polymer may cause a plate-out of the THEIC on injection molding surfaces. Over time this plate out effect will result in the cessation of molding so that the molds can be cleaned. Additionally, the molded parts themselves may have a sticky or tacky surface. One attempt at solving bloom problems in intumescent flame retardant systems containing APP is described in U.S. Pat. No. 3,936,416 to Brady.
There still remains a need, however, for improved flame retardants which can be added to thermoplastic polymer systems. It is, therefore, an object of this invention to provide improved flame retardants suitable for use in thermoplastic polymer systems, including homopolymers of polyolefins and copolymers, terpolymers, et cetera of one or more polyolefins. It is also an object of this invention to provide flame retardants and polymer systems containing such flame retardants which result in thermoplastics having improved processability, including the reduction of screw slippage, the reduction of mold plate-out, and the reduction of surface bloom problems. It is yet another object of this invention to provide flame retardants and polymer systems containing such flame retardants which result in thermoplastics having good mechanical properties. These and other objects of the invention will become apparent from the description of the invention.